Planets census: Earth may not be unique

SAN JOSE - Nearly one in four stars like the sun could be supporting Earth-size planets, according to the first- ever planets census of our celestial neighbors.

Although these planets have not yet been detected, a mathematical calculation by University of California-Berkeley astronomers Geoff Marcy and Andrew Howard concludes that Earth may not be unique.

"When I go out alone at night, and look into the night sky, my face becomes flushed at the idea that those twinkling lights have other planets around them - and some are small, perhaps the size of Earth," Marcy said.

Through telescopes at San Jose's Lick Observatory and Hawaii's Keck Observatory, they gazed at 166 nearby stars - many visible to the naked eye, such as 55 Cancri in the constellation Cancer and Tau Ceti in Cetus - and detected the presence of 33 giant planets, the size of Jupiter and Neptune, around 22 of these stars.

Because small planets are believed to be more abundant than large planets, they used an extrapolation to conclude that 23 percent of these stars could have Earth-sized planets.

"We have the first census of planets of different sizes," Marcy said. "We projected, for the first time, that there are planets not much bigger than Earth, orbiting nearby stars."

Last month, a decade-long hunt by UC-Santa Cruz astronomer Steven Vogt yielded the discovery of a planet, called Gliese 581g, that could be the most Earth-like planet ever found - and the best case yet for a habitable one, a possible end to our cosmic loneliness.

The new paper, published in today's issue of the journal Science, takes a much broader and less focused sweep of the skies.

It examines stars within 80 light-years of Earth. Already, experts from the Search for Extraterrestrial Intelligence (SETI) are focusing their hunt for radio and television signals from these specific stars.

Looking deeper into the universe, astronomers believe there may be far more Earth-size planets orbiting stars much farther away. They estimate the universe is home to 1 septillion stars (a 1 with 24 zeros), boosting the likelihood that some of them hold "Goldilocks" planets, neither too hot nor too cold, and potentially habitable.

"The tantalizing evidence suggests there are more Earths," Marcy said.